The present invention relates to plasma arc torches and, more particularly, to a method of forming an electrode for supporting an electric arc in a plasma arc torch.
Plasma arc torches are commonly used for the working of metals, including cutting, welding, surface treatment, melting, and annealing. Such torches include an electrode which supports an arc which extends from the electrode to the workpiece in the transferred arc mode of operation. It is also conventional to surround the arc with a swirling vortex flow of gas, and in some torch designs it is conventional to also envelop the gas and arc with a swirling jet of water.
The electrode used in conventional torches of the described type typically comprises an elongate tubular member composed of a material of high thermal conductivity, such as copper or a copper alloy. The forward or discharge end of the tubular electrode includes a bottom end wall having an emissive element embedded therein which supports the arc. The element is composed of a material which has a relatively low work function, which is defined in the art as the potential step, measured in electron volts (ev), which permits thermionic emission from the surface of a metal at a given temperature. In view of its low work function, the element is thus capable of readily emitting electrons.when an electrical potential is applied thereto. Commonly used emissive materials include hafnium, zirconium, tungsten, and their alloys. Some electrodes include a relatively non-emissive separator, which is disposed about the emissive element and acts to prevent the arc from migrating from the emissive element to the copper holder.
A problem associated with torches of the type described above is the short service life of the electrode, particularly when the torch is used with an oxidizing gas, such as oxygen or air. More particularly, the gas tends to rapidly oxidize the copper of the electrode that surrounds the emissive element, and as the copper oxidizes, it more readily emits. As a result, a point is reached at which the oxidized copper surrounding the emissive element begins to support the arc, rather than the element. When this happens, the copper oxide and the supporting copper melt, resulting in early destruction and failure of the electrode.
Many conventional electrodes are assembled by pressing the emissive insert into the metallic holder, or by pressing the emissive insert into a relatively less- or non-emissive sleeve or separator that is then pressed into the metallic holder. The interfaces between the press-fit emissive element, separator, and holder are relatively well defined, and thereby negatively affect the thermal conductivity of the assembled electrode. Specifically, heat travelling through the electrode encounters the interfaces, which act as barriers to heat transfer and thus restrict the heat transfer ability of the electrode. In addition, the well defined interfaces act as stress concentrators that may attract the arc and accelerate the demise of the electrode.
In order to help xe2x80x9csmoothxe2x80x9d the interfaces between the emissive element, separator, and holder, the assignee of the present invention has developed a diffusion bonding technique described in a co-pending application with Ser. No. 09/773,847 (xe2x80x9cthe ""847 applicationxe2x80x9d) entitled xe2x80x9cElectrode Diffusion Bonding.xe2x80x9d In the co-pending ""847 application, a post-assembly heating step is described that creates a diffusion bond between the separator and the metallic holder. The diffusion bond softens or smoothes the interface between the two materials, while increasing the bond strength therebetween. As a result, the electrode has a longer operational life.
Another method of forming an electrode is described in another co-pending application with Ser. No. 09/871,071 (xe2x80x9cthe ""071 applicationxe2x80x9d) entitled xe2x80x9cElectrode Interface Bonding.xe2x80x9d In the co-pending ""071 application, an intermetallic compound is formed between the emissive element and the separator that provides an improved bond therebetween. The intermetallic bond is formed by heating the emissive element and separator to about 1700xc2x0-1800xc2x0 F. for about 1 hour. A second post-fabrication heating step can also be performed in order to form a eutectic bond between the separator and the metallic holder.
While the post-assembly heating step of the co-pending ""847 and ""701 applications represent improvements in the state of the art, further improvements are desired. In particular, a study of the materials used in an electrode shows that many electrodes employ an emissive element comprising hafnium, zirconium, or the like; a separator comprising silver, gold, nickel, or the like; and a metallic holder comprising copper. While the post-assembly heating steps of the co-pending ""847 and ""701 applications improve the bonds between the emissive element and the separator, and between the separator and the holder, it is desirable to further improve the bond therebetween or provide an advantageous alternative.
The present invention was developed to improve upon conventional and recently discovered methods of making electrodes. It has been discovered that the deficiencies in the life and performance of electrodes for plasma torches can be improved by forming the electrode by inserting an emissive element in a molten or substantially flowable non-emissive material and then allowing them to cool to form an assembly that is used to form the electrode. Advantageously, the emissive element and non-emissive material form a strong bond therebetween relatively quickly, and in, some cases an intermetallic compound is formed between the emissive element and the non-emissive material or member, which in one embodiment acts as a separator between the emissive element and the metallic holder. As such, the electrode of the present invention performs better and can be manufactured faster than conventional electrode fabricating methods.
In particular, a method of fabricating an electrode according to the present invention includes heating a relatively non-emissive material or member until the material becomes substantially flowable. The heating step is preferably performed with a crucible, where the non-emissive material is heated in the crucible to at least around its melting point. The emissive element is positioned above the melted non-emissive material and the emissive element is allowed to drop or is advanced at least partially into the non-emissive material. Force can also be used to help advance or insert the emissive element into the non-emissive material. This position is then held for a predetermined time, such as about 1 minute, and the assembly is allowed to cool. In one embodiment, a vacuum or reduced pressure environment is created around the emissive element and the non-emissive material during the fabrication of the electrode. As a result, an intermetallic compound is formed between the emissive element and the non-emissive material, which provides a superior bond therebetween compared to diffusion bonds or interference fits.
According to one embodiment, the assembly formed by the emissive element and the non-emissive material is then positioned in a conventional metallic holder of the electrode. The positioning step may include press fitting, brazing, or welding the assembly into the holder. Alternatively, the assembly itself may be shaped to define the front of the electrode instead of including the metallic holder. Thus, the non-emissive member of the assembly can be shaped to define a substantial portion of the electrode. This is advantageous because the non-emissive member is preferably formed from a material that has greater thermal conductivity than conventional materials that form the metallic holder. For example, the non-emissive material used to replace the metallic holder is preferably formed from at least one of the materials from the group consisting of silver, gold, platinum, rhodium, iridium, palladium, nickel, monel, and alloys thereof, while metallic holders are typically formed from copper.
In yet another embodiment, the metallic holder is used a the xe2x80x9ccruciblexe2x80x9d for the relatively non-emissive material wherein a blank of the non-emissive material is heated in a cavity defined by the metallic holder or blank until it becomes substantially flowable. The emissive element is then inserted or advanced into the non-emissive material, preferably during the heating step. The metallic holder is then shaped into a predetermined shape so that the electrode can be used in cutting and welding operations.
Advantageously, the intermetallic compound formed between the emissive element and the non-emissive material is formed faster than that described in the co-pending ""701 application. More specifically, the intermetallic compound forming step according to the present invention can be performed on the order of about 1 minute, while the intermetallic compound forming step according to the ""701 application occurs on the order of about 1 hour. Therefore, the electrode fabrication process of the present invention provides a significant time savings while still achieving an improved bond between the emissive element and the non-emissive member compared to conventional methods.